The present invention relates generally to frequency control systems and, more particularly, to a frequency control system operative in a receiver which receives signals transmitted in intermittent bursts.
A communication system is comprised, at a minimum, of a transmitter and a receiver interconnected by a communication channel. A communication signal is transmitted by the transmitter upon the transmission channel to be received by the receiver. A radio communication system is a communication system in which the transmission channel comprises a radio frequency channel defined by a range of frequencies of the electromagnetic frequency spectrum. A transmitter operative in a radio communication system must convert the communication signal into a form suitable for transmission upon the radio-frequency channel.
Conversion of the communication signal into a form suitable for transmission upon the radio-frequency channel is effectuated by a process referred to as modulation. In such a process, the communication signal is impressed upon an electromagnetic wave. The electromagnetic wave is commonly referred to as a "carrier signal." The resultant signal, once modulated by the communication signal, is commonly referred to as a modulated carrier signal. The transmitter includes circuitry operative to perform such a modulation process.
Because the modulated carrier signal may be transmitted through free space over large distances, radio communication systems are widely utilized to effectuate communication between a transmitter and a remotely-positioned receiver.
The receiver of the radio communication system which receives the modulated carrier signal contains circuitry analogous to, but operative in a manner reverse with that of, the circuitry of the transmitter and is operative to perform a process referred to as demodulation.
Numerous modulated carrier signals may be simultaneously transmitted upon differing radio frequency channels of the electromagnetic frequency spectrum. Regulatory bodies have divided portions of the electromagnetic frequency spectrum into frequency bands, and have regulated transmission of the modulated carrier signals upon various ones of the frequency bands. (Frequency bands are further divided into channels, and such channels form the radio-frequency channels of a radio communication system.)
A two-way radio communication system is a radio communication system, similar to the radio communication system above-described, but which permits both transmission and reception of a modulated carrier signal from a location and reception at such location of a modulated carrier signal. Each location of such a two-way radio communication system contains both a transmitter and a receiver. The transmitter and the receiver positioned at a single location typically comprise a unit referred to as a radio transceiver, or more simply, a transceiver.
A two-way, radio communication system which permits alternate transmission and reception of modulated carrier signals is referred to as a simplex system. A two-way radio communication system which permits simultaneous transmission and reception of communication signals is referred to as a duplex system.
A cellular communication system is one type of two-way radio communication system in which communication is permitted with a radio transceiver positioned at any location within a geographic area encompassed by the cellular communication system.
A cellular communication system is created by positioning a plurality of fixed-site radio transceivers, referred to as base stations or base sites, at spaced-apart locations throughout a geographic area. The base stations are connected to a conventional, wireline telephonic network. Associated with each base station of the plurality of base stations is a portion of the geographic area encompassed by the cellular communication system. Such portions are referred to as cells. Each of the plurality of cells is defined by one of the base stations of the plurality of base stations, and the plurality of cells together define the coverage area of the cellular communication system.
A radio transceiver, referred to in a cellular communication system as a cellular radiotelephone or, more simply, a cellular phone, positioned at any location within the coverage area of the cellular communication system, is able to communicate with a user of the conventional, wireline, telephonic network by way of a base station. Modulated carrier signals generated by the radiotelephone are transmitted to a base station, and modulated carrier signals generated by the base station are transmitted to the radiotelephone, thereby to effectuate two-way communication therebetween. (A signal received by a base station is then transmitted to a desired location of a conventional, wireline network by conventional telephony techniques. And, signals generated at a location of the wireline network are transmitted to a base station by conventional telephony techniques, thereafter to be transmitted to the radiotelephone by the base station.)
Increased usage of cellular communication systems has resulted, in some instances, in the full utilization of every available transmission channel of the frequency band allocated for cellular radiotelephone communication. As a result, various ideas have been proposed to utilize more efficiently the frequency band allocated for radiotelephone communications. By more efficiently utilizing the frequency band allocated for radiotelephone communication, the transmission capacity of an existing, cellular communication system may be increased.
The transmission capacity of the cellular communication system may be increased by minimizing the modulation spectrum of the modulated signal transmitted by a transmitter to permit thereby a greater number of modulated signals to be transmitted simultaneously. Additionally, by minimizing the amount of time required to transmit a modulated signal, a greater number of modulated signals may be sequentially transmitted.
By converting a communication signal into discrete form prior to transmission thereof, the resultant modulated signal is typically of a smaller modulation spectrum than a corresponding modulated signal comprised of a communication signal that has not been converted into discrete form. Additionally, when the communication signal is converted into discrete form prior to modulation thereof, the resultant, modulated signal may be transmitted in short bursts, and more than one modulated signal may be transmitted sequentially upon a single transmission channel.
As a single frequency channel is utilized to transmit two or more separate signals during nonoverlapping time periods, a method of signal transmission is referred to as a time division method. A communication system incorporating such a time division method of signal transmission includes a Time Division Multiple Access communication system or, more simply, a TDMA communication system.
A TDMA communication system includes a transmitter operative to transmit signals to a receiver in intermittent bursts during intermittent time periods. Such signal transmitted to a particular receiver operative in a TDMA communication system shall hereinafter be referred to as a TDMA signal.
A TDMA communication system is advantageously utilized as a cellular communication system as, during time periods in which a base station does not transmit a TDMA signal to a particular radiotelephone, another TDMA signal may be transmitted to another radiotelephone at the same frequency as the frequency at which the TDMA signal transmitted to the first radiotelephone is generated. Thus, a single base station may transmit signals to two or more radiotelephones at the same frequency. It should be noted that, typically, a base station generates signals at a particular frequency continuously, but a TDMA signal is referred to as the portions of the continuously-generated signal generated by the base station directed to a particular radiotelephone.
A receiver operative to receive a TDMA signal need only be powered during the intermittent time periods during which the TDMA signal is transmitted to the receiver. During other time periods, the receiver need not be powered. By properly synchronizing the receiver of a TDMA communication system with a transmitter which transmits the TDMA signal thereto, the receiver may be powered only during those time periods in which the TDMA signal is transmitted to the receiver, and the receiver may be turned-off (i.e. the receiver may be disconnected from a supply of power) during other time periods. Significant power savings can result as the receiver need only be powered to receive the signal transmitted thereto for one-half, or an even smaller fraction, of the time period that a receiver operable to receiver a conventional signal must be powered. (As a receiver operable to receive a conventional, continuously-generated signal must be powered continuously, a conventional receiver may not be turned-off.)
As many radiotelephones operable in a cellular communication system are powered by battery power supplies, power savings resulting from intermittent powering of the receiver portion thereof increases the amount of time in which the radiotelephone may be operated by a single power supply. As a result, a cellular communication system operable to transmit and to receive TDMA signals is also advantageous for this reason.
However, conventional frequency control circuitry which forms a portion of a receiver and which is operative to prevent frequency drift of the receiver requires a signal to be applied continuously to the frequency control circuitry for proper operation thereof. Conventional frequency control circuitry, accordingly, inadequately prevents frequency drift of a receiver which is only intermittently powered to receive a TDMA signal. (Frequency control circuitry of a receiver compares the frequency of a signal transmitted thereto with the frequency of one or more oscillators forming a portion of the receiver. Responsive to such comparison, the oscillation frequency of the receiver-oscillator is altered.)
Receivers constructed to receive digital signals (whether transmitted only in intermittent bursts or continuously) oftentimes include decision-directed demodulators/signal detectors. Such decision-directed devices are capable of providing a frequency control signal which may be utilized by frequency control circuitry for purposes of frequency control and do not require application of a continuous signal thereto. However, such frequency control signals generated by the decision-directed devices are accurate only when the frequency of the transmitted signal is closely related in frequency with that of the receiver oscillator.
Frequency control utilizing conventional frequency control circuitry (when a continuously-generated signal is received) is operable to correct for frequency differences between a transmitted signal and the oscillation frequency of an oscillating signal generated by a receiver oscillator as great as, or even greater than, ten times the range of frequency differences over which frequency control circuitry utilizing a decision-directed device is operable. (For example, conventional frequency control circuitry is operable when the frequency differences are as great as 30 kilohertz while frequency control circuitry including the decision-directed devices is operable over frequency differences up to only approximately 3 kilohertz.)
Because of this limited range of operability, frequency control circuitry utilizing the decision-directed device cannot be utilized when the frequency differences between the signal transmitted to the receiver and the oscillation frequency of a receiver-oscillator is significant, such as immediately subsequent to initial powering of the receiver prior to effectuation of frequency control.
Accordingly, what is needed is frequency control circuitry operative in a receiver when a signal is transmitted thereto in intermittent bursts even when frequency differences between such transmitted signal and the receiver-oscillator, oscillation frequency is significant.